LEO 15/17 BH41 00268A - Intermitent loss of one color

[F u r u y á]

Monitor: Samsung SyncMaster 795MB

MODEL NAME: LEO 15/17Inch(53~85Khz)
REV NO.: MP1.2(2004.04.12)
CODE NO.: BH41-00268A

Manuals: [link (single .zip), mirror (4 parts .rar)]


Problem: Intermitent loss of one color (I don't remeber which one)

Diagnosis: It looks like the problem is a damaged solder in the neck board (when I slightly push the neck board in one side, the problem goes away).


So I want to be able to inspect the solders of the neck board but I can't do it because of the shield [see pictures: left, right, top, back], which is soldered (big solders btw) to the neck board in 5 points apparently (4 corners and center).



My questions are:

1) To be able to inspect the solders, I have to desolder the shield from the neck board, right?

2) Do I have to remove the neck board itself? If so, what's the procedure?



Thanks in advance

[MonMotha]

1) To be able to inspect the solders, I have to desolder the shield from the neck board, right?
2) Do I have to remove the neck board itself? If so, what's the procedure?

1) Pretty much.  It's often easiest to just reflow them all on something as small as the neckboard rather than trying to isolate a single fault.  Connectors and large transistors are common culprits for bad solder joints, thogh.

2) Not really, but it's way easier if you do and less risky in terms of tube damage.  It comes off easily, too.  There's usually a little bit of silicone holding the tube socket on the board onto the neck of the tube.  This can be cut with a sharp knife, or it will usually separate from the crt socket if you're gentle when pulling.  Other than that, just pull straight back, and it should come right off the tube.

There's also going to be a ground line running from the braid that's strapped to the back of the CRT to the neckboard that you may need to remove in order to get the neckboard into a work area.  The plug type varies but usually just pulls off.  Just don't forget to put it back on when you re-attach the neckboard.

[F u r u y á]

In the pictures, you can see that there's a green plastic belt around the socket. So I have to cut it before trying to pull the neck board off the tube?

I never did the reflow technique (I have knowledge/experience only with soldering). Do you think that in my case it'is worth to buy the tools/materials and try to reflow the whole neck board or should I just stick to finding the defective solder and remake it?

[MonMotha]

Didn't notice your pictures.  The green "belt" is probably not attaching anything, but there does appear to be a white clip up top that you may need to release somehow.

And I don't mean actually reflowing the board like in an oven.  You actually can't do that on the phenolic material usually used in monitors as it will break.  I just mean hit everything with an iron to bring the solder to a molten state then let it cool back down.  If you get an unusually dull joint, you may consider sucking most of the existing solder off and putting new, quality solder back down.

[F u r u y á]

Here is the green belt: link. It's a hard plastic and thick. I took some pictures that show that it looks like it goes under the socket aswell so I'm afraid it's part of the locking mechanism? I will post the pictures soon. * EDIT * Pictures: [1, 2]

That belt is very similar to the black one in this picture (from this thread).


Thanks for the (fast!) reply

[F u r u y á]

Finally I managed to pull it off. The green thing is indeed part of the socket.

After some hours trying to remove the shield I finally removed it too (damn I need a third arm! ). And... after a quick look I couldn't find defective solders. Just tested the RGB connector (the one that plugs into the neck board) and VGA connector (continuity test, bending in the RGB wires in every direction) and it looks ok, no bad contacts. Looks like I will have to make a lot of tests.


Is it ok to test the monitor with the neck board without its shield?

[F u r u y á]

Well, I tested it anyway.

I connected GND connectors directly to the ground line of the neck board. Originally, the GND connectors are connected to the shield which is soldered to GND line of the board.


The monitor is now 99% repaired

After looking carefully for bad solders in the Blue signal path [because I assumed that the screen was yellowed (didn't quite remeber the color )] with the aid of the Schematic and PCB diagrams, I've found two very sneaky broken solder in the pins of a IC.


When I resolder the shield and put back the case, it will be 100%. 

[F u r u y á]

Problem not solved!

I'm getting intermitents cyan screen (no red signal) when I do little taps in the case. Now I'm too lazy to yet again reopen the case, remove neck board, desolder shield...

[MonMotha]

Well, check for bad solders everywhere.  That's the normal cause.

You *may* have a CRT fault, which probably isn't worth fixing given the relative ease with which one can acquire CRT PC monitors these days.  If you happen to know someone with/have a CRT tester, you can check for cathode shorts.

[F u r u y á]

I will, but later. I'll let you know.


Meanwhile, thanks for the help MonMotha, really appreciated.